Conventionally used solid-state image sensing devices are broadly divided into a CCD type and a MOS type, depending on the means for reading an optical charge generated at a photoelectrically converting element. The CCD type transfers the optical charge while accumulating it in a well of a potential, whereas the MOS type reads, through MOS transistors, the charge stored in a p-n junction capacitance of a photodiode. However, since an output proportional to the amount of the generated optical charge is produced, the dynamic ranges of these conventional solid-state image sensing devices are narrow.
U.S. Pat. No. 5,241,575 to Minolta discloses a solid-state image sensing device capable of outputting electric signals converted natural-logarithmically to the intensity of the incident light by providing (to increase the dynamic range), photosensitive means capable of generating photoelectric current proportional to the intensity of the incident light, a MOS transistor supplied with the photoelectric current, and bias means for biasing the MOS transistor so that a subthreshold current can flow therethrough. This solid-state image sensing apparatus has insufficient low-light condition and SN ratio characteristics, despite having a wide dynamic range.